1. Field of the Invention
The present invention relates to a data communications system in which the data is communicated between the host computer and the terminal equipment via a packet switching network.
2. Description of the Prior Art
The packet switching system is a well-known communications system which will now be described briefly.
Data transferred through the packet switching network is divided into packets of a predetermined length, each having a header and a data block of a predetermined length necessary for being transferred in the network. The packet is stored in the packet switch-board and is then sent to the destination equipment.
Equipment connected to the packet switching network include the following two types. That is, one is an equipment which is called a packet mode device having a function (which is referred to as packet function here) which assembles the data that is to be transferred by itself into a packet and which disassembles the received packet and converts it into a series of data, and the other is an equipment called a non-packet equipment (NP-DTE) and which by itself does not have the above-mentioned packet function. Terminal equipment generally pertains to NP-DTE.
The packet switching network consists chiefly of a packet switch-board having a packet switching function, and a packet multiplexer (PMX) having a packet assembly and disassembly function (hereinafter referred to as PAD) which assembles or disassembles the packet for the NP-DTE and which executes the transfer control operation (protocol conversion which corresponds to a procedure which is contained in the packet mode but which is not contained in the non-packet terminal, or which corresponds to a procedure contrary thereto) like the packet mode equipment, the packet multiplexer further includes a traffic concentration function for multiplexing a plurality of packets on a transmission line.
When it is desired to receive the service of a host computer which is a packet mode equipment through a terminal which is NP-DTE such as a start-stop terminal, it is necessary to apply for subscription so that the general terminal is permitted to receive the service of the PAD and to register an identifier for the equipment. The data sent from the NP-DTE that receives the service from the PAD is compiled into a packet by the PAD and is sent to the host of the packet mode. Further, the packet sent from the host of the packet mode is disassembled and is transferred to the NP-DTE. The PAD further exchanges the PAD message necessary for the packet switching system relative to the packet mode host.
Thus, the service of the host of the packet mode is received from the NP-DTE through the packet switching network.
The communication procedure between the PAD and the packet mode equipment is executed according to a protocol which is called X.29 recommended by CCITT.
The above-mentioned packet switching system has been described in detail in such literatures as "A HANDBOOK OF DATA COMMUNICATIONS", compiled by the Japanese Association of Electronic Communications, published by Ohm Co., pp. 204-206, and "INTERFACE OF PACKET SWITCHING SERVICE", compiled by NTT Co., published by the Association of Nippon Data Communications, pp. 183-192 and 435-443.
Here, the packet mode host computer is called PAD service host which is connected to the PAD and which is receiving a packet from, or which is giving a packet to, the PAD according to the protocol X.29, i.e., which is carrying out the service relative to the PAD.
Here, it is presumed that a user of the NP-DTE connected to a given computer system is trying to receive a data base service by making access to a packet mode host computer that is connected to the packet switching network and that is carrying out the data base service.
In this case, the user is required to apply for a subscription so that the NP-DTE is permitted to receive the service of the PAD, and to connect it to the PAD.
Japanese Patent Laid-Open No. 131148/1982 teaches to connect to the packet switching network a packet mode host computer to which a plurality of NP-DTE's are connected. The communications are controlled between the packet mode equipment and the packet switching network according to the protocol X.25 recommended by CCITT. Therefore, the packet mode host computer is allowed to exchange the data relative to other packet mode equipment connected to the packet switching network according to the protocol X.25, but is not allowed to receive the service of the host computer which is carrying out the data base service according to protocol X.29 through the NP-DTE that is connected to the packet mode host computer. The service of the host computer is simply received through the NP-DTE which is connected thereto.
Therefore, even though there exists a computer system that includes a plurality of terminals that are connected to the host computer and that utilizes TSS (time sharing system), the users of the terminals who wish to receive the service of other host computers connected to the packet switching network must individually apply for the subscription and connection of terminal equipment to the PAD of the packet network.